The universally-recognized best complete modern recording of Wagner's Tristan und Isolde is the 1966 live recording at Bayreuth conducted by Karl Böhm with Birgit Nilsson as Isolde and Wolfgang Windgassen as Tristan. There also is an excellent one from the so-called "Golden Age" (1936) with the Royal Opera House, Covent Garden, having Fritz Reiner as conductor, Kirsten Flagstad as Isolde, and Lauritz Melchior as Tristan. Both recordings are readily available. Some here might quibble with me on these choices, but everyone has their preferences.
Wade
P.S. - Don't limit yourself to T & I. As soon as you've gotten your fill of this particular opera, come back for some more advice. However, at this point, I don't exactly recommend that you go from T & I to Der Ring des Nibelungen, but maybe you might want to further "wet your feet" by listening to orchestral excerpts from the other operas. George Szell's Sony recordings of such repertoire have been considered a standard for years, though other conductors such as Ormandy, Bernstein, Klemperer, and Walter made their recorded contributions to this particular repertoire. This is probably the best way to familiarize yourself with Wagner's music that will be very useful to you as a learning tool and recognition media for the coming years.